In etching a substructure partly covered with a film (or films) resistant to the etchant, a portion of the substructure below the lateral edge of the film is typically also removed. This is referred to as underetching since an open space is created below the edge of the film. U.S. Pat. No. 3,721,592 discloses a technique to suppress underetching which is normally undesirable.
Underetching can sometimes be put to good use. For example, the silicon substructure in U.S. Pat. No. 3,958,040 is underetched below a patterned multilayer film at the edges of the field area where a recessed oxide-isolation region is to be formed. A second multilayer film is then formed along the surface of the structure, including the surface of the open space below the overhanging portion of the first film. A layer of positive photoresist is deposited. Part of the photoresist goes into the open space below the overhanging part of the films. The photoresist is now exposed to ultraviolet (UV) light. The films shield the photoresist in the open space from the UV light. During the subsequent development step to remove the exposed photoresist, the unexposed photoresist in the open space remains.
The structure is subjected to an etchant that removes the uncovered part of the second film. The photoresist occupying the open space protects the underlying segment of the second film during this step. After removing the remainder of the photoresist, a high-temperature oxidation treatment is employed to grow the oxide-isolation region. Due to the remaining segment of the second film, the height of the bird heads that grow at the edges of the field area is markedly reduced. The underetching of the silicon substructure therefore leads to a positive result.